Human temporal bones obtained at autopsy from patients with various forms of sensorineural deafness are to be studied by microdissection and phase-contrast examination of surface preparations. The role of strial atrophy, vascular pathology, and degeneration of the spiral ligament will be evaluated in relation to hair cell loss and degeneration of cochlear nerve fibers in a) presbycusis b) ototoxicity, and c) noise-induced deafness. Similar studies will be carried out in monkeys treated with ototoxic antibiotic and other drugs, for which complete behavioral audiograms have been obtained before, during, and after treatment. Similar work will be carried out in monkeys and guinea pigs exposed to intense noise, and to combinations of noise and ototoxic drugs. The parameters of cochlear injury and of synergism and/or potentiation between ototoxic drugs and noise will be studied in guinea pigs by measuring cochlear potentials and correlating these with the location and extent of hair cell loss; and in monkeys by behavioral audiometry and hair cell counts. Inner ear pathology observed by light microscopy will also be examined by transmission and scanning electron microscopy. In presbycusic human ears and in temporal bones of aging dogs, vestibular as well as cochlear changes will be evaluated, including otoconial and neuroepithelial degeneration. Mechanisms involved in the formation of the otoconia will continue to be investigated in gerbils injected with Ca45. The innervation of the cochlear and vestibular hair cells will be studied in guinea pigs by silver and zinc iodide-osmic acid staining methods.